During manufacturing of printed products, measures are typically taken to ensure a certain level of printing quality. This is particularly true in the field of security printing where the quality standards that must be reached by the end-products, i.e. banknotes, security documents and the like, are very high. Quality inspection of printed products conventionally involves optical inspection of the printed product using suitable camera systems for acquiring images of the printed material. Such optical inspection can be performed as an off-line process, i.e. after the printed products have been processed in the printing press, or, more and more frequently, as an in-line process, i.e. directly on the printing press where the printing operation is carried out.
Inspection methodologies for inspecting printed products, especially security documents, are for instance disclosed in U.S. patent publication Nos. U.S. Pat. Nos. 5,384,859 and 5,317,390.
Various types of in-line inspection systems are known in the art including systems which make use of matrix-type cameras, i.e. cameras comprising array sensors that are designed to take snapshots of the entire surface of the printed material to be inspected, or line-scan cameras, i.e. cameras comprising linear sensors that are designed to scan the entire surface of the printed material to be inspected while the printed material is being moved relative to the camera.
FIGS. 1 and 2 show a prior art sheet-fed intaglio printing press for printing sheets of securities, especially banknotes, comprising an in-line inspection system for inspecting the quality of the sheets being printed on the intaglio printing press.
Reference numeral 01 designates a feeding station for feeding successive sheets to a downstream-located printing group 02 of the intaglio printing press where the sheets are printed one after the other.
As is common in the art of intaglio printing, the printing group 02 comprises an impression cylinder 10 which cooperates with an intaglio printing cylinder 11 carrying engraved intaglio printing mediums on its circumference. More precisely, in this particular example, the intaglio printing cylinder 11 consists of a three-segment plate cylinder carrying three intaglio printing plates on its circumference, which intaglio printing plates are mounted on the cylinder 11 by suitable plate clamping systems (not shown) disposed in corresponding cylinder pits 11a (see FIG. 2). In this particular example, the impression cylinder 10 exhibits the same diameter as the intaglio printing cylinder 11 and carries three impression blankets on its circumference, which blankets are secured on the impression cylinder 10 by corresponding holding systems (not shown) located in cylinder pits 10a of the impression cylinder 10 (see again FIG. 2).
The intaglio printing cylinder 11 is inked by a suitable inking system comprising, in this example, an indirect inking system and a direct inking system. The indirect inking system includes an ink-collecting cylinder (or “Orlof cylinder”) 12 contacting the intaglio printing cylinder 11 and three colour-selector cylinders 13 and associated inking units (not referenced) for applying multicolour ink patterns onto the ink-collecting cylinder 12. The direct inking system includes a fourth colour-selector cylinder 14 which is in direct contact with the intaglio printing cylinder 11 and an associated inking unit (not referenced) for applying an additional ink pattern onto the circumference of the intaglio printing cylinder.
In this example, all four inking units which supply ink to the colour-selector-cylinders 13, 14 are disposed in a mobile inking carriage 20 that can be retracted away from the printing group 02 to a maintenance position 20* (as shown in dashed lines in FIGS. 1 and 2).
Prior to printing, the inked intaglio printing mediums are wiped by a suitable wiping system comprising, as is typical in the art, a wiping roller 15 contacting the circumference of the intaglio printing cylinder 11. Excess inks which have been applied outside of the engravings of the intaglio printing mediums are removed under the action of the wiping roller 15 which also forces and pushes the inks into the engravings to achieve proper printing quality.
This particular intaglio printing configuration is purely illustrative and other configurations are known in the art, for instance from US patent publications Nos. U.S. Pat. Nos. 4,516,496, 5,062,359, 5,899,145, 7,011,020 B2 (US 2004/0237816 A1) and US 2007/0181016 A1, all in the name of the present Applicant, the contents thereof being incorporated herein by reference.
Sheets are fed in succession to the impression cylinder 10 by the feeding station 01, which impression cylinder 10 rotates in the clockwise direction in the illustrations and transports each sheet in succession past the printing nip formed between the impression cylinder 10 and the intaglio printing cylinder 11. Once printed, the sheets are taken away from the impression cylinder 10 by a suitable sheet conveyor system 03 for delivery to a sheet delivery station 04 comprising multiple delivery piles. The sheet conveyor system 03 consists, in this particular example, of a chain gripper system typically comprising two endless chains supporting a plurality of space-apart gripper bars (not shown) for holding the printed sheets by a leading edge thereof, which endless chains are driven along a delivery path (which runs in the counter-clockwise direction in the illustrations) between two pairs of chain wheels 31, 32.
FIGS. 1 and 2 further show that an in-line inspection system 05 and associated sheet-guiding member 06 are disposed along the path of the conveyor system 03, as well as a drying and/or curing unit 07.
The in-line inspection system 05 of FIGS. 1 and 2 and associated sheet-guiding member 06 are designed according to the disclosure of US patent publication No. US 2005/0127595 A1 (corresponding to International publication No. WO 03/070465 A1) which is incorporated herein by reference in its entirety. Other known examples are disclosed in U.S. patent publication Nos. U.S. Pat. No. 5,329,852, US Re. 35,495, U.S. Pat. Nos. 6,111,261, 6,176,482 B1.
A particularity of the inspection system 05 and sheet-guiding member 06 resides in the fact that the inspection system 05 comprises a matrix-type camera for taking snapshots of the freshly printed surface of the sheets (which freshly printed surface is oriented downwards in this example), while the backside of the sheets is being held against a curved suction surface of the sheet-guiding member. Examples of images taken by such an inspection system are for instance disclosed in FIGS. 4 and 4A to 4C of International publication No. WO 2007/060615 A1.
Other solutions for carrying out in-line inspection of printed sheets on intaglio printing presses are disclosed in U.S. patent publication Nos.US Pat. No. 6,746,014 B2 (US 2002/0108516 A1), U.S. Pat. No. 6,772,689 B2 (US 2002/0035939 A1), and US 2007/0175912 A1. Like the aforementioned disclosures, these other solutions are also based on the use of a matrix-type camera for acquiring images of the printed sheets to be inspected.
The use of line-scan cameras for performing in-line inspection of printed material on printing presses is also known as such in the art (see for instance US patent publications Nos. US 2009/0007807 A1 and US 2009/0025594 A1). Such line-scan cameras are in particular advantageous in that they are typically available with better performance and higher resolutions as compared to matrix-type cameras. Line-scan cameras are also advantageous in that they allow to take images that are substantially free of any optical aberrations or deformations. It is furthermore typically easier to ensure illumination consistency over the entire inspected surface using line-scan cameras as one only has to ensure an adequate illumination of the linear portion of the printed material being inspected, rather than of the entire sheet.
The use of line-scan cameras for performing in-line inspection of printed material on intaglio printing presses has however and so far not been put into practice due to constraints that are inherent to the intaglio printing process and to the behaviour of intaglio printing presses.
Indeed, intaglio printing is in particular characterized by very high printing pressures applied between the impression cylinder 10 and the intaglio printing cylinder 11. When the cylinder pits 10a, 11a of these cylinders 10, 11 meet, characterizing shocks are generated which produce vibrations that propagate to the entire printing press. It has therefore been typically considered that line-scan cameras would not be suitable to perform in-line inspection on intaglio printing presses as the vibrations and shocks resulting from the operation of the printing press would interfere with the image acquisition process of line-scan cameras and create optical aberrations and errors in the images taken by such systems.
In practice, matrix-type cameras have therefore been believed to be the only type of cameras suitable for performing in-line inspection on intaglio printing presses.
European patent publication No. EP 0 323 537 A1 discloses an intaglio printing press equipped with a device for detecting a defective print based on the inspection of the inked surface of the intaglio printing medium prior to printing. This device comprises a line-scan type CCD camera disposed so that its lens faces the surface of the intaglio printing cylinder at a position located downstream of the wiping roller with respect to a rotational direction of the intaglio printing cylinder in order to acquire an image of the inked surface of the intaglio printing mediums prior to printing. Besides the fact that the device is not as such inspecting the printed result and cannot therefore detect print defects that may occur as a result of the printing operation, this solution is also affected by the shocks and vibrations created by the cooperating impression cylinder and intaglio printing cylinder which will propagate to the line-scan camera and interfere with the image acquisition process.
European patent publication No. EP 2 230 202 A2, which was published only after the relevant priority date of the instant application discloses a quality inspection apparatus for an intaglio printing press which makes use of multiple line-scan cameras for acquiring an image of the printed sheets along the path of the printed sheets which are transported to the delivery station by means of a sheet conveyor system of the type comprising endless chains driving spaced-apart gripper bars. This publication is totally silent about the issues pertaining to shocks and vibrations that propagate throughout the intaglio printing press during the printing operation and that affect proper operation of the line-scan cameras. In view of the fact that the solutions disclosed in European patent publication No. EP 2 230 202 A2 make use of three line-scan cameras directed at different locations of the printed sheets, it is inevitable that the shocks and vibrations propagating through the intaglio printing press will interfere with the image acquisition process of the line-scan cameras.